As well known in the art, a hybrid drive unit comprises an electric motor or a motor generator as an extra power source in addition to an internal combustion engine. Therefore, the hybrid drive unit is capable of reducing emission from the internal combustion engine and improving fuel economy, by driving the internal combustion engine as efficiently as possible, while adjusting excess and deficiency of output torque and engine braking force by the electric motor or the motor generator, and regenerating energy during deceleration. According to the drive unit of this kind, the electric motor capable of functioning as a generator being connected with a power distribution mechanism generates electric power when an output speed is relatively low. The electric power generated by the electric motor is supplied to another electric motor thereby operating said another electric motor as a motor, and a power of said another electric motor is added to an output member. To the contrary, in case the output speed is raised to relatively high speed, the electric motor connected with the power distribution mechanism is used as a motor by rotating the electric motor in a direction opposite to a rotational direction of the internal combustion engine. The output of the electric motor being rotated as a motor is used to rotate said another electric motor thereby operating said another electric motor as a generator. That is, the output of the electric motor functioning as a motor is converted into an electric power by said another electric motor functioning as a generator, and the converted electric power is returned to the electric motor of an input side to be converted into a mechanical power again and transmitted to an output side. Thus, the output of the electric motor is converted between mechanical power and electric power, and such power circulation induces undesirable energy losses. As a result, transmission efficiency is deteriorated and fuel economy of hybrid vehicle is thereby degraded.
In order to solve the problem explained above, conventional hybrid drive unit have been adapted to set at least two drive modes by varying a substantial speed change ratio between an internal combustion engine and an output member. One example of such hybrid drive unit is disclosed in Japanese Patent Laid-Open No. 2004-345527. According to the teachings of Japanese Patent Laid-Open No. 2004-345527, the driving device of hybrid car comprises a first motor generator arranged coaxially with a rotational center of an engine, a single pinion type planetary gear mechanism, a double pinion type planetary gear mechanism, and a second motor generator. The engine and the first motor generator are connected through the single pinion type planetary gear mechanism functioning as a power distribution mechanism. A carrier of the single pinion type planetary gear mechanism is connected with a ring gear of the double pinion type planetary gear mechanism, and a ring gear of the single pinion type planetary gear mechanism is connected with a carrier of the double pinion type planetary gear mechanism. The ring gear of the single pinion type planetary gear mechanism and the carrier of the double pinion type planetary gear mechanism are connected with the output member and the second motor generator. This driving device of hybrid car further comprises a brake for halting a sun gear of the double pinion type planetary gear mechanism.
Therefore, according to the driving device of hybrid car taught by Japanese Patent Laid-Open No. 2004-345527, a normal mode, in which an output torque of the engine and a torque of the first motor generator synthesized or distributed by the single pinion type planetary gear mechanism is/are outputted to the output member, and in which an engine speed is controlled by the first motor generator, is set in case the brake is being released. To the contrary, in case the sun gear of the double pinion type planetary gear mechanism is halted by engaging the brake, the double pinion type planetary gear mechanism functions as a speed increasing mechanism in which the output element is rotated at high speed using the sun gear as a fixing element, ring gear as an input element and carrier as an output element. As a result, the engine speed can be lowered relatively even if the output speed is high. On the other hand, Japanese Patent Laid-Open No. 2005-9514 discloses a control system fixing a planetary mechanism to which an engine and a first motor generator are connected to function as a speed increasing mechanism.
According to the teachings of Japanese Patent Laid-Opens Nos. 2004-345527 and 2005-9514, power of the engine is distributed to the output member such as an output shaft by two sets of the planetary gear mechanisms or complex planetary gear mechanisms, and a drive mode in which the power is transmitted to the output member while converting the power and a drive mode in which the power is transmitted to the output member mechanically are set by engaging or releasing the brake. Thus, the power distribution mechanism is substantially composed of a plurality of planetary gear mechanisms, and a quantity of planetary gear mechanisms are therefore required to form the hybrid drive unit. For this reason, size of those hybrid drive unit have to be enlarged and mountability of those hybrid drive units on a vehicle is thereby degraded.